A jealous rival could be a likely betrayer. Maybe someone who is envious of Bell's popularity or his relationship with certain characters. For example, another adventurer who feels overshadowed by Bell might betray him to gain an advantage. Another possibility is a character who is being manipulated by a darker force. They might not have wanted to betray Bell originally but are forced to under the influence of some evil power.
It could be someone from Bell's own party. In some fanfictions, a member of his group might betray him due to a personal grudge or a difference in goals. Perhaps they feel that Bell's actions are putting the whole party in danger, and in a moment of panic or anger, they turn on him. Or, it could be a character who has been secretly working for an enemy all along and finally decides to make their move and betray Bell.
Another possibility is that it could be an old acquaintance who has changed for the worse. They might have been jealous of Ash and Lyra's success or popularity in the Pokémon world. This person could use their knowledge of Ash and Lyra's habits and weaknesses to betray them. It could also be a group of people rather than just an individual, like a team of thieves or poachers who see Ash and Lyra as obstacles to their illegal activities.
A possible betrayer could be a jealous demigod. There are always those in the Percy Jackson world who envy Percy's abilities and the attention he gets. This demigod might see an opportunity to get rid of Percy and make themselves look better in the eyes of the other campers.
It could be a fellow student who was secretly jealous of Harry all along. Maybe someone like Draco Malfoy. He had always been at odds with Harry and in a fanfiction, he could be pushed to the point of betraying Harry for his own self - centered reasons. Another possibility is a teacher. A teacher who was influenced by the dark side or had some hidden agenda could betray Harry. For example, if there was a teacher who was secretly working for Voldemort or a new dark wizard.
Gary Oak could be a likely betrayer. He has always been Ash's rival, and in some fanfiction, he might take things too far and betray Ash in a crucial Pokémon battle, like using underhanded tactics to win. This would surely make Ash cry.
It could be some of the pure - blood supremacists who were biding their time. Even though the war was over, they might still resent Harry for his role in defeating Voldemort and decide to get revenge by making it look like he betrayed them. Another possibility is some of the Ministry officials who were corrupt and didn't want Harry's influence around.
One common theme could be the exploration of Bell's emotional turmoil after the betrayal. It might show his struggle to trust again. Another theme could be the consequences for the one who betrays Bell, perhaps facing the wrath of his friends or the Gods. Also, the rebuilding of Bell's relationships in the face of such a betrayal can be a theme. Fanfic writers might use this as an opportunity to show Bell's growth and resilience as he overcomes the pain of being betrayed.
Another type of betrayer could be a family member. In some fanfictions, family dynamics can be complex. A family member might betray fem Percy out of a sense of duty to a different cause or because they believe she is a threat to the family's reputation or safety.
One aspect that can be explored in Freya and Bell fanfiction is the idea of destiny. Freya believes Bell is destined for great things, and she wants to be a part of that journey. A fanfiction could show how she subtly influences Bell's choices in the dungeon, leading to both growth and conflict. Bell might start to question why certain opportunities seem to present themselves so conveniently, and as he discovers Freya's role, he has to decide whether to accept her help or forge his own path.
Luke could be a likely candidate in some fanfictions. He already had his own issues and motives in the original story, and it wouldn't be a stretch for a fanfiction writer to take that further and have him betray Percy in a more elaborate way.
Well, Bell is obviously one of the most popular. He's the main character in the original series and in the fanfiction, his growth and adventures are always the focus. People like to read about how he overcomes new challenges in the game - like setting.